午餐會報 (2024)

Fast Radio Bursts (FRBs) have been shown to have diverse properties and may come from very different host environments. To better understand the FRBs' origin, multi-wavelength detailed studies of a large sample of nearby FRB hosts will provide valuable information. To fulfill this goal, BURSTT has been designed with a large field of view and sub-arcsecond localization capabilities. In this talk, I will introduce the BURSTT project and its development status, focusing on the backend system, calibration, and VLBI timing I mainly work on.

Star formation in galaxies is governed by the amount of molecular gas and the efficiency that gas is converted into stars. However, assessing the amount of molecular gas relies on the CO-to-H2 conversion factor (α_CO), which is known to vary with molecular gas conditions like density, temperature, and dynamical state – the same conditions that also alter star formation efficiency. The variation of α_CO, particularly in galaxy centers where α_CO can drop by nearly an order of magnitude, thus causes major uncertainties in current molecular gas and star formation efficiency measurements. Using ALMA observations of multiple 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that α_CO is primarily driven by CO opacity changes and therefore shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio. Motivated by these results, we have established a new α_CO prescription which accounts for CO emissivity variations and verified it across a set of nearby galaxies with independent α_CO measurements from dust. Applying our new prescription to 65 galaxies from the PHANGS-ALMA survey, we found an overall 3x higher star formation efficiency in barred than non-barred galaxy centers, which is an unprecedented trend that has been obscured by past α_CO prescriptions. Our results suggest that the high star formation rates observed in barred galaxy centers is mainly due to an enhanced star formation efficiency, rather than a substantially increased amount of molecular gas.

Protostellar jets often exhibit asymmetric properties between both lobes, ranging from their morphological and kinematic features to gas properties such as density and temperature. To determine whether these differences in the surrounding medium or the intrinsic properties of the jet, we need to characterize and trace these asymmetries close to the source. The Th 28 protostellar jet highlights these features with striking asymmetries between the jet lobes, while also showing a small-scale wiggling with a point symmetry indicating precession. I will present results from an optical/NIR study of this jet which combines integral field spectroscopy from VLT/MUSE and echelle spectroscopy from VLT/X-Shooter to obtain a 3D spatial and spectral view of both jet lobes, allowing us to investigate both the asymmetries and symmetries between them.

Ten years ago I was living in Taipei as a postdoc at ASIAA. Early the following year I left to try my luck in the field of data science, which involved studying, taking on side projects to build a portfolio, and applying for jobs. Ultimately, I took a job in the field of online education. Since then I've worked with three different companies to create courses focused on artificial intelligence and machine learning that serve thousands of learners worldwide. In this talk I will include a brief summary of my work in astronomy followed by stories and learnings from my transition into the world of silicon valley ed-tech companies.

Statistical X-ray AGN studies show that a significant fraction of active galactic nuclei (AGN) show large amounts of absorption in their X-ray emission. The fraction of absorbed AGN also shows an increase with redshift and a decrease with luminosity. However, these trends are not well established above redshift 2, especially for luminous AGN (log Lx [erg /s]>44.5). Using the uniquely deep & wide multiwavelength imaging datasets in the HSC-Deep XMM-LSS field, we investigated the absorbed fraction of luminous AGN with log Lx [erg /s]>44.5 above redshift 2. We found an absorbed fraction of 76±4% which is more than twice the fraction in the local universe (~30%). We further investigated the incidence of absorption in samples of Type 1 & 2 AGN at redshift 0.8-1.8 under AGN obscuration scenarios which describe absorption properties in the local universe. For most of the sample, the Eddington ratio of Type-2 AGN is lower than type-1 broad-line AGN. The distribution is consistent with dusty nuclear obscuration regulated by radiation pressure. However, we find evidence of non-nuclear obscuration among the sample of Type-1 AGN. These observations indicate that high redshift black hole growth occurs under heavy obscuration which is not fully explained under models of nuclear obscuration by a dusty torus. It requires additional components of obscuration from the host galaxy as well as dust-free absorption by nuclear gas.

Understanding how material accretes onto a rotationally supported disk from the surrounding envelope of gas and dust in the youngest protostellar systems is important for describing how these disks are formed. Recently, many observations have confirmed the existence of so-called "streamers", which are extended filamentary-like structures feeding a reservoir of mass to these protostellar disks. We present one such case of a unique Class 0 protostar, Lupus 3-MMS, which shows multiple infalling streamers along the edge of the outflows in C18O. We isolate these streamers using dendrograms and compare to a simple model of infalling trajectories, which matches the observations very well (>96% in four out of five of the dendrogram structures). We derive several properties of the streamers, such as their mass and infall rate, and compare them in the context of other confirmed streamers in Class 0 protostars and values from MHD simulations of protostellar disk formation. Our results confirm the dynamically infalling nature of multiple streamers in Lupus 3-MMS and show that even these simple models can be good approximations when compared to observations.

Quasars (QSOs) – active super-massive black holes at the center of galaxies – are the brightest non-transient sources in the Universe and are powered by intense accretion episodes. The copious radiation emitted by a luminous QSO can, like a flashlight, illuminate the surrounding material, allowing us to directly study structures extending to circumgalactic (~100 kpc) and intergalactic (Mpc) scales. In this talk I will quickly report on some of the latest results of the QSOMUSEUM survey which comprises VLT/MUSE observations for 120 z~3 single quasar fields and 8 quasar pairs. Using a high-resolution cosmological simulation I will then showcase the importance of the QSO radiation on circumgalactic scales, and show how the observation of extended emission around quasars not only give us access to the gas properties, but also to the properties of the central engine itself (black-hole mass, accretion rate, ionization cone opening angle).

Magnetic fields (B-fields) are believed to play an essential role in the formation and evolution of interstellar clouds and protostars. A vast amount of polarization data from thermal dust emission has made significant progress in studying B-fields and dust in star-forming regions. In this talk, I will present our studies on B-fields toward several molecular clouds using SOFIA/HAWC+ and JCMT/Pol-2. Then, I will focus on probing dust physics, including grain alignment (RAT-A) and rotational disruption (RAT-D) due to radiative torques (RATs), toward several filaments. Our numerical modelings using RAT theory can successfully reproduce maps of polarization fraction from observational data. Our results can explain the decrease of the polarization fraction toward the dense regions (aka. polarization holes), provide more robust evidence for the RAT paradigm, and insights into dust grains.

Planet formation mechanisms are intimately tied to the structure of protoplanetary disks and the properties of dust grains. The first theme of the talk focuses on dust settling, which may serve as a prerequisite for triggering streaming instability by enhancing the midplane dust-to-gas ratio in the protoplanetary disks. Utilizing high angular resolution observations from ALMA, I present vertically resolved, edge-on disks around HH 212 mms and IRAS 04302+2247. The results demonstrate that the dust has not completely settled yet in these young systems. The second theme attempts to understand the grain sizes using submillimeter continuum polarization. While dust scattering from spherical/randomly aligned grains can explain much of the polarization patterns, I show that scattering of aligned grains can simultaneously explain the multiwavelength polarization pattern for HL Tau and also the polarization substructure.

Thousands of other worlds beyond the Solar System (i.e., exoplanets) have been found. In this informal talk, I shall give the audience a glimpse of my view of exoplanet populations based on hydrogen isotopes. Interestingly, my view may be undermined by recent JWST observations and updated statistical analyses of these other worlds.

The physical properties of protostellar disks, such as mass and size, are likely closely related to the planet formation process. Several mechanisms have been proposed to influence disk properties during star formation. To observationally examine these theories, we have been measuring disk properties and comparing them with magnetic fields, turbulence, gas kinematics, and stellar mass in various systems. In this presentation, I will summarize our observational results on the relationships between disk properties and physical conditions in protostellar sources, discuss key mechanisms determining disk properties, and demonstrate that theoretical predictions from the combination of hydrodynamics and non-ideal MHD can explain the observed size distributions.

Until the early 2000s, uniprocessor performance climbed along with Moore's law, doubling approximately every two years. To obtain further speedups with computer programs, one had to only purchase a new machine. However, microprocessor manufacturers hit a fundamental wall, which meant that clock frequencies could not be increased further with mass-produced processor coolers. After this turning point, sequential programs had to be parallelized to obtain further speedups. The free lunch was over. Today, graphics processing units are the flagships of massively parallel accelerators, which have been used successfully to accelerate machine learning, computational chemistry, and physics by an order of magnitude compared to general-purpose purpose processors traditionally used in high-performance computing. In this talk, we will introduce the concepts of implementing physical simulation software on graphics processing units, discuss the outlook of the high-performance computing landscape, and give an overview of the state-of-the-art methods for accelerating astrophysical simulations.